# Can this planet have an ocean of water?

1. Apr 23, 2013

### willstaruss22

Lets say there is an exoplanet with 0.5 Earth masses and is 0.8 Earth radii. This planet has an atmospheric pressure of 0.10 bar/10% as thick as Earths atmosphere with an average surface temperature of 285 K. Can this planet have a body of liquid water the size of an ocean with the low atmospheric pressure?

2. Apr 24, 2013

### Staff: Mentor

Saturation pressure at that temperature is ~16mbar, a significant fraction of your atmosphere would be H2O.

The escape velocity at the surface is roughly 10km/s. This is similar to earth, indicating that the lifetime of the atmosphere is similar as well. Some hydrogen will be lost, but water can be present for billions of years. A magnetic field can help against solar wind, but it is not necessary, as Venus shows.

3. Apr 24, 2013

### willstaruss22

So how large a fraction are we talking, over 10 of the total mass of the atmosphere?
Could an ocean as large as the Indian Ocean exist?

4. Apr 24, 2013

### snorkack

You gave the volume fraction on surface. 285 K means 16 mbar saturated water vapour. Since the total is 100 mbar, it means 84 mbar is other gases.

This is the volume fraction. What the mass fraction is depends, on surface, what the molecular mass of the other gases is. Total mass of the atmosphere, depends also on the climate. The upper levels of atmosphere will be less than 16 % water, because some will rain or snow back down, but exactly how much less depends on the details of atmospheric composition and climate.

Sure. An ocean bigger than Pacific could also exist. The water fraction is arbitrary, in a wide range.

5. Apr 24, 2013

### willstaruss22

Well my planet will have 73% nitrogen, 8% oxygen and 2% Co2 factoring the 16% water vapor. I was kind of going for simple life that is resistant to solar radiation producing oxygen in the atmosphere. I was expecting there to be 50% land 50% water on the planet.

Last edited: Apr 24, 2013
6. Apr 24, 2013

### Staff: Mentor

Should be fine.
The density is slightly higher than Earth's, indicating more heavy elements, and probably more radioactive decays in the core. It looks like plate tectonics is possible.

If oxygen is produced, something (living or not) should consume oxygen again.

7. Apr 24, 2013

### willstaruss22

Now as far as rotation is was going for 12 hours and going for a tilt of 28 degrees but im not sure how the climatic condition of these two will act in a 0.10 atmosphere. My thinking is the shorter days will even up the temperature differences to an extent and that the more extreme tilt will even the difference between the equator and poles.

8. Apr 24, 2013

### Staff: Mentor

More tilt will lead to more extreme seasons at the poles, in addition to the colder climate there.

9. Apr 24, 2013

### willstaruss22

Ok so do you think that with the conditions presented the liquid oceans could last a few billion years?

Last edited: Apr 24, 2013
10. Apr 25, 2013

### snorkack

Mars, with 0,1 Earth masses, had running rivers, and probably oceans, for a long time. It seems likely that a planet slightly more massive than Mars could keep its water.

11. Apr 25, 2013

### willstaruss22

Ok so you said that there could be an ocean the size of the pacific ocean on this planet. So here is a question what about a global ocean with no land at all still keeping the atmospheric pressure at 0.10? I have heard that regular oceans and seas can exist in liquid form with a atmosphere like this so what about a total global ocean?

12. Apr 25, 2013

### Staff: Mentor

There is no limit on the size of the ocean (well, it cannot cover more than 100% of the surface :p). This is independent of the atmosphere - if oceans are possible, their size is not restricted.